With the engine, transmission, and rear-wheel drive, the 2005 Volkswagen Jetta has been averaging 18.57 MPG (miles per gallon).

The 2005 Volkswagen Jettas in this analysis were driven mostly in the city (about 42.7 percent of the miles driven). The rest of the miles were on the highway at an average speed of 113 miles per hour (24.7 percent), in heavy traffic (21.3 percent), and in the suburbs (0 percent).

In addition, the average 2005 Volkswagen Jetta in this analysis was driven on flat terrain with a light foot and the AC on only some of the time.

Even when driving extremely conservatively, I have never been able to get much more than 30 mpg highway on this car, which seems low for a car of this size and power. A taller overdrive might help in this regard.
see full Volkswagen Jetta review

2005 Volkswagen Jetta Gas Mileage (MPG)

Unlike other fuel economy surveys, TrueDelta's Real-World Gas Mileage Survey includes questions about how and
where a car was driven. So you can get an idea of the Volkswagen Jetta's
real-world MPG based on how and where you drive a car.